1. Field of the Invention
The present invention relates to composite materials containing diamond. The instant invention also relates to applications of such composite materials, particularly as armors for defeating ballistic projectiles.
2. Introduction
2.1 Goals and Approach:
Current military personnel armor systems consist of two primary components, namely a vest (often referred to as “soft armor”) and plates (often referred to as “hard armor”) that fit within pockets in the vest. The plate portion of the system is most typically a lamination of a ceramic tile and a rigid fiber-reinforced polymer backing. The purpose of the ceramic tile is to damage the bullet, and the purpose of the fiber-reinforced polymer backing is to support the ceramic and to catch fragments of the damaged bullet. The microstructure and properties of the ceramic tile can greatly affect the performance of the system with respect to single-shot performance, multi-hit performance and field durability.
2.2 Executive Summary:
The present research that is the basis for this patent document was part of a larger series of US government funded research programs. The primary goal of these programs was to develop enhanced personnel armor systems by improving the hard component in the assembly. Key improvement needs were increased durability, increased mass efficiency, and the ability to defeat more aggressive threats. Tool steel-tipped threats were studied, as well as the even more aggressive armor piercing (AP) threats. The present research project was solely focused on the “tile” in the armor system. Thus, a constant backing system (armor design) was used for all testing.
The approach used to achieve the objective was:                Conduct materials development/optimization experiments using 4-inch (100 mm) square plates as the test size. Experiments evaluated all appropriate “tile” materials.        Evaluate 4-inch (100 mm) square tiles for:                    Microstructure, composition and properties.            Single-shot ballistic performance                        In parallel, evaluate microstructure, properties and ballistic performance of current commercial personnel armor tile materials. Results obtained on these materials provide a benchmark for assessing success on the program.2.3 Projectiles and Phase Transformations:        
To provide comparative information, various projectile cores were obtained and examined for microstructure and hardness. Results are provided in Table 1 and FIG. 1. Referring to FIG. 1 in particular, the left hand micrograph is of a 7.62 mm “M993” round, and the right-hand micrograph is of a 7.62 mm “New Lenox” test round. In light of the work of Viechnicki et al. [1], where it was stated that an effective armor ceramic must be harder than the projectile, it is important to use the results in Table 1 as a guide when conducting materials development work. That is, one must ensure that the material being developed is harder than the threat of interest.
TABLE 1Properties of ProjectilesHardness*,Hardness*,kg/mm2kg/mm2CoreDensity,(Knoop Scale,(Knoop Scale,ProjectileCompositiong/cc500 g load)2 kg load)7.62 mm Soft Steel7.8 202 ± 10Out of RangeLPS20 mm Soft Steel7.8 394 ± 18Out of RangeFSP7.62 mm Tool Steel7.8739 ± 9720 ± 5APM214.5 mm WC/Co14.81503 ± 631434 ± 20BS-417.62 mm WC/Co14.81278 ± 241185 ± 44M9937.62 mm NewWC/Co14.81490 ± 821410 ± 17Lenox Test Round*Testing performed on a Shimadzu HMV-2000 microhardness tester. Measurements taken at both 500 g and 2 kg to ease comparison with data in literature.
In addition to hardness concerns, one must be aware of possible pressure induced phase transformations when developing hard armor materials. High density projectiles, such as WC/Co, a composite of tungsten carbide and cobalt metal, commonly referred to as “cemented carbide”) apply very high pressure upon impact. To effectively defeat such projectiles, the armor material must be capable of sustaining the pressure load without a phase transformation. This phase transformation-like effect led to a decrease in ballistic performance. Information in the literature suggests that Si and B4C will degrade when exposed to the high pressures applied by WC/Co projectiles upon impact. Specifically, as shown in Tables 2 and 3, the pressure applied by a WC/Co projectile upon impact is higher than the threshold pressure at which Si and B4C fail. Silicon fails via a pressure induced phase transformation [2], and B4C fails via an “amorphization” [3].
TABLE 2Pressures Applied at Impact by 7.62 mm Projectiles Constructed ofTool Steel and WC/CoProjectilePressure Applied byConstructionProjectile During MuzzleProjectile TypeMaterialVelocity Impact (GPa)7.62 × 54 R mm B32Tool Steel~15 [ref. 4]7.62 × 51 mm NATO FFVWC/Co~23 [ref. 4]
TABLE 3Threshold Pressures for Phase Transformations in Si and B4CPressure at Which Phase MaterialTransformation Occurs (GPa)ReferenceSi9-162B4C~203
To yield desired performance levels versus threats that apply a high pressure load, the Phase II research focused on reducing or eliminating the Si and B4C contents of reaction bonded ceramics.
2.3 Ballistic Testing:
Ballistic testing was conducted using two different 7.62 mm armor piercing (AP) threats. One threat, the APM2, has a core of tool steel. As a result of its modest density, the pressure it applies at impact is insufficient to cause phase transformation of Si and/or amorphization of B4C. The second threat, M993, has a core of WC/Co. At muzzle velocity, this high density projectile applies sufficient pressure to promote phase transformation and/or amorphization of Si and B4C. An optimized material that successfully defeats both of these threats will have broad utility in armor applications.
Materials screening ballistic tests were conducted on 100 mm×100 mm (4 inch square) ceramic tiles (single shot testing). These tests generated a V50 result in accordance with MIL-STD-662 (six or more tiles per test set). The tiles were backed with a polymer composite to yield a “system”. For all tests, the areal density of the ceramic tile and the polymer backing were held constant.
2.4 Other Test Methods:
In addition to ballistic test data, various physical and mechanical property results are presented within the following sections. A summary of test methods used to collect the data are provided in Table 4. In addition to these tests, various specialized tests are used throughout the R&D activities. When appropriate, these tests are described within the text.
TABLE 4Test Methods Utilized for Physical and Mechanical PropertiesPropertyReferenceTest MethodDensityASTM C135-86Water ImmersionYoung's ModulusASTM E494-95Ultrasonic VelocityPhase Fractions—Quantitative Image Analysis(using Clemex VisionLite)Composition (compounds)—X-Ray Diffraction (XRD)Composition (elements)—Glow Discharge MassSpectrometry (GDMS)Knoop HardnessASTM C1326-99Diamond IndentationFlexural StrengthASTM C1161-90Four-Point Bend TestFracture ToughnessMunz et al. [7]Four-Point Bend ChevronNotch Test
Unless stated otherwise, the Knoop hardness testing was performed with a 2 kg load. This follows the Army's recommendation for hardness characterization of armor ceramics [8].
2.5 Reaction Bonded Ceramics, including those for Armor Applications:
The production of reaction bonded ceramics has been described elsewhere in detail [5, 6]. In short, the process consists of two primary steps. First, a porous mass or preform of ceramic particles (e.g., SiC or B4C) plus carbon is fabricated. Second, the preform is reactively infiltrated with molten Si. During the infiltration process, the Si and carbon react to form SiC, which bonds the ceramic particles into an interconnected network (hence the name reaction bonding). The process is shown schematically in FIG. 2.
The body thus formed features the porous mass material, typically silicon carbide, distributed throughout the in-situ silicon carbide formed from the chemical reaction. Typically, some infiltrant material remains in the infiltrated body, and distributed throughout. The process by which such “reaction bonded” ceramics or composites is made is called “reaction-bonding”, although other terms have been used in the literature over the years to mean substantially the same thing. These terms include “reaction forming”, “reaction sintering”, and “self bonding”.
A variation of the reaction bonding process is described in U.S. Pat. No. 3,951,587 to Alliegro et al. Here, it was discovered that molten silicon can spontaneously wick into a porous mass of silicon carbide without the need for elemental carbon to be present to react with the molten silicon, although the process may require somewhat higher temperatures than the reaction bonding process, and may still not be as robust. Many refer to the product resulting form this process as “siliconized SiC”, and to the process by which it is made as “siliconizing.”.
A primary feature of the reaction bonding process is that the pore space in the preform is filled by infiltration. Thus, nominally no volume change occurs during processing. This is very different when compared to the sintering and hot pressing processes where the pore space is closed by shrinkage of the part—typically 20% linear shrinkage. Because there is virtually no shrinkage, the reaction bonding process allows the production of large and complex shapes. Another feature of reaction bonded ceramics is the fact that molten Si, like water, expands upon solidification. Therefore, a finished reaction bonded ceramic is fully dense. Fully dense microstructures are desired in armor applications.
The microstructure and properties of M Cubed Technologies' currently fabricated reaction bonded silicon carbide, or “RBSC” (grade SSC-802) and reaction bonded boron carbide, or “RBBC” (grade RBBC-751) ceramics are provided below in FIG. 3, and Tables 5 and 6. These data, which are from actual production tiles, are used as a baseline to which results from experimental tiles made on the program are compared.
TABLE 5Properties of M Cubed Technologies' Current Armor-Grade Reaction BondedSilicon Carbide Ceramic (Grade SSC-802)PropertyResultCommentsDensity (g/cc)3.00 ± 0.02SPC Data - population of >5,000 tilesYoung's 365 ± 3SPC Data - population of Modulus (GPa)>5,000 tilesKnoop 2 kg 1332 ± 116>10 Test TilesHardness(kg/mm2)Flexural Strength 290 ± 25>10 Test Tiles(MPa)Fracture Toughness 4.0 ± 0.5>10 Test Tiles(MPa-m1/2)Calculated Si Content: Composition 23.9 vol. %via RuleSiC Content: of Mixtures76.1 vol. %Quantitative XRDSi: 16.7 wt. % (21.6 vol. %)SiC: 83.3 wt. % (78.4 vol. %)
TABLE 6Properties of M Cubed Technologies' Current Armor-Grade Reaction BondedBoron Carbide Ceramic (Grade RBBC-751)PropertyResultCommentsDensity (g/cc)2.56 ± 0.02SPC Data - population of>5,000 tilesYoung's 392 ± 6SPC Data - Modulus (GPa)population of >5,000 tilesKnoop 2 kg Hardness 1626 ± 127100 Test Points(kg/mm2)Flexural Strength (MPa)304 ± 3535 Test TilesFracture Toughness 4.5 ± 0.235 Test Tiles(MPa-m1/2)Quantitative Image Si Content: 13.4 vol. %AnalysisCeramic Content: 86.6 vol. %Quantitative XRDSilicon: 13.3 wt. %Silicon Carbide: 8.7 wt. %Boron Carbide: 78.0 wt. %
In summary, reaction bonded ceramics offer many advantages for armor applications. They have attractive properties, can be formed in complex shapes, and can be economical to produce. However, issues do exist. For one, the hardness of the current form of reaction bonded SiC is below that of some potential WC/Co threats (discussed above). Thus, a hardness improvement is needed. In addition, as was previously stated, some phases in these ceramics are not suited to the dynamic, high pressure load applied by high density projectiles. Modification to the formulation is necessary.
3. Citation of Related Art: Composite Materials Containing Diamond
The following is a list, not guaranteed to be exhaustive or inclusive, of patents that may be relevant to the present invention:    U.S. Pat. No. 4,220,455 to St. Pierre et al.    U.S. Pat. No. 4,453,951 to Ohno    U.S. Pat. No. 7,008,672 to Gordeev et al.    U.S. Pat. No. 6,955,112 to Adams et al.